The present invention relates generally to television receivers and, in particular, to a circuit for dynamically adjusting the value of the black level clamp voltage of a DC restoration circuit, such adjustment circuits commonly being referred to as DC coupling reduction circuits.
The video component of a transmitted television signal is characterized by a DC level determining the average background brightness of the televised scene. In order for the reproduced image at the television receiver to have the proper background brightness, i.e. the proper level with respect to black, the transmitted DC component must be retained or otherwise restored in the receiver. In television receivers employing direct coupling techniques throughout the entire video section, the DC component of the composite video signal is inherently maintained by the receiver and no special circuits are necessary to insure the reproduction of a picture having a proper average background brightness level. However, for purposes of simplicity and economy, as well as to prevent the accummulation of system errors, most modern television receivers have video sections incorporating RC coupling circuits. As a result of the DC blocking action of the coupling capacitors in these circuits, the background brightness determining component of the video signal is normally lost or reduced and must therefore be suitably restored in the receiver to enable reproduction of an acceptable picture.
Various forms of DC restoration circuits have been devised for accomplishing the foregoing function of restoring the DC level of the video component of a transmitted television signal. One such DC restoration circuit is disclosed in copending application Ser. No. 118,127 filed Feb. 4, 1980, now U.S. Pat. No. 4,293,874, and assigned to the assignee of the present invention. Another DC restoration circuit is illustrated in an article entitled "A Versatile Low-Level Luminance IC for TV" appearing on pages 169-175 of the August 1978 issue of IEEE Transactions on Consumer Electronics, Vol. CE-24, No. 3. In both of these circuits, a viewer operable brightness control is used to establish a selected black level in the form of a DC reference potential which is coupled to a feedback network for clamping the back porch of the horizontal blanking interval of the video signal to the selected reference potential. Thus, in response to adjustment of the brightness control, the viewer may adjust the black level to a desired setting. However, once the brightness control is set to a selected position corresponding to a particular reference potential, the back porch of the horizontal blanking interval remains fixedly clamped to this potential independent of the average or DC level of the associated video signal. A DC restoration circuit operating in this manner, i.e. where the back porch is clamped to a fixed level, is said to be characterized by 100% DC coupling.
In certain instances it has been found desirable to decrease the amount of DC coupling from the 100% value to a somewhat lower value typically on the order of about 85%. This reduction in the level of DC coupling permits a dynamic adjustment in the clamping level of the back porch which serves to compensate for broadcast transmission inconsistencies by slightly decreasing the average background brightness of a highly white scene and by increasing the average background brightness of a dark scene. In the past, such DC coupling reduction circuits have typically taken the form of a resistive voltage divider connected in parallel with a relatively large value capacitor and inserted directly in the video signal transmission path. Circuits of this type are considered undesirable in modern television receivers where integrated circuit technology finds widespread use since additional integrated circuit chip pins must be provided to enable connection of the circuits.
It is accordingly a basic object of the present invention to provide a novel DC coupling reduction circuit for dynamically adjusting the clamping level of a DC restoration circuit in accordance with the average DC level of a video signal. It is a further object of the invention to provide a DC coupling reduction circuit which may be incorporated on an integrated circuit chip together with a DC restoration circuit without necessitating the inclusion of additional integrated circuit chip pins.